The invention refers to a process for the extended use of electrolytes, especially for depositing of non-glare metal coatings on a metal surface.
It is understood that metal coatings also include metal alloy coatings, and metal surfaces also include metallized surfaces of non-metallic objects.
The electrolytes used for plating generally are based on the customary, known composition components for the particular metal being plated. For example, in the case of nickel deposits from Watts nickel type electrolytes, these would be nickel sulfate, nickel chloride and boric acid.
Electrolytes based on chloride, sulfate, sulfamate, fluoroborate or mixtures thereof may be used for the metals nickel, copper, zinc, tin as well as for noble metals. In addition, the electrolytes may contain selected organic or inorganic base brighteners, grain improvers, brighteners or complexing agents.
In order to achieve absence of glare, a finely dispersed system is generated in the electrolyte by the addition of inorganic or organic foreign materials, so that these additives during the precipitation are either built into the deposit and/or interrupt for a short time the precipitation of the cathode film locally.
In the case of the known processes, finely ground insoluble materials, such as graphite, barium sulfate, aluminum oxide, glass etc. are added to the electrolyte. These products are maintained in suspension by strong agitation of the electrolyte and built into the deposit during the precipitation.
In modern processes the fine dispersal phase is generated in the electrolytes themselves. One or more surfactants are added to the solution which form a homogeneous solution below their cloud point, but form a finely dispersed emulsion above their cloud point.
As is the case with all emulsions, the stability of the fine dispersal phase is limited. Over time, the droplets increase in size and negatively affect the uniformity of the precipitation.
In practice, the emulsion is stabilized in that an electrolyte is continuously cooled to below the cloud point temperature, whereby the surfactant again goes into complete solution in the electrolyte. Subsequently, the electrolyte is again brought to the working temperature. The heating/cooling circuit required causes substantial additional operating cost.
If one works with an elevated salt content, the addition of a few milligrams per liter of selected surfactants already are sufficient to achieve a non-glare precipitation. Even without the forming of a visibly, cloudy emulsion, high-quality glare suppression is achieved. The surfactant may subsequently be readjusted several times, so that a worsening of the appearance of the layers due to coagulation occurs only after a period of 12-14 hours. The electrolyte functions without a secondary circuit. The coagulate may be removed by filtration over activated charcoal.
In addition, a fine dispersal phase may be generated by the reaction of soluble anionic and cationic combined with difficult-to-dissolve, higher-molecule salts in electrolytes. However, also in this case, after a working cycle of 8-10 hours the foreign material must be removed completely from the electrolytes by filtration due to coagulation effects. For the next working cycle the dispersion phase must be rebuilt again each time by the addition of components.
The higher-molecular compounds may also be directly added to the electrolytes.
Materials such as proteins or polysaccharides are present in aqueous solutions in colloidal form, and they lead to non-glaring coatings with suitable electrolyte compositions. These compounds are reduced by hydrolysis and/or reaction at the electrodes, and the glare removing property is increasingly destroyed.
The invention deals with the technical problem to prepare a process for the extended use of electrolytes which ensures a high electrolyte quality with a low economic investment, without having to accept production interruptions for regeneration.
Especially, it is envisaged to conduct the process for the precipitation of non-glaring metal coatings in such a manner that work can proceed in three shifts, five days per week without encountering production interruptions due to coagulation or destruction of the fine dispersal phase, respectively, and without the additional operating cost of a heating/cooling circuit.
The solution proposed by the invention is that during the continued use of an electrolyte a part of the flow is deviated, filtered and, if necessary, reconstituted by the addition of active substances and subsequently reintegrated in the operating cycle.
It is therefore suggested, in a partial flow of the electrolyte, to continually remove by filtration, wholly or in part, the coagulated particles and the not yet coagulated particles, as well as high-molecular compounds and their decay products, and that the appropriate quantity of additives is added to the filtrate prior to its reintegration.